The invention provides a solution to practical problems encountered in implementing a "cross-eye" system. "Cross-eye" is an electronic countermeasure which is described, e.g. in D. C. Schleher, Introduction to Electronic Warfare (Artech House, Dedham, Mass., 1986). Typically, a "cross-eye" system has two pair of antennae widely separated from one another. The members of each pair are located close to one another and each member of each pair is connected by a waveguide to a corresponding member of the other pair. Thus, a signal received at one of the pairs of antennae is conducted to the other pair, and retransmitted (repeated). For a "cross-eye" system to work, signals received and repeated must be phase-tracked, i.e. for a signal received at each pair of antennae a time T apart, the signal would emerge and be repeated at the other pair of antennae spaced by the same time T. Stated alternatively, phase shifting of one signal with respect to the other along the waveguides must be virtually zero (other than phase shifting intentionally imposed: cross-eye systems conventionally shift the signal in one waveguide 180.degree. so that the repeated signals null along the line of sight of incoming signals).
To produce this virtually zero phase shifting, the electromagnetic path of each waveguide must be virtually identical. This has proved difficult in practice, particularly at higher frequencies, where extremely small distortions in one waveguide would introduce significant changes in effective path length. For example, routing of the waveguide through a crowded environment necessitates the penetration of many ship water-tight bulk heads with the associated costs of cutting moderately large openings and welding at each penetration. This approach also involves the assembly of many piece parts with a result of a diminished system reliability caused by the requirement that all of the assembled pieces must remain attached at the proper tension for the waveguides to operate. The use of waveguides and/or coaxial cable also causes installation difficulties because both of these have moderate bend radius constraints. Thus, this approach was found to be difficult, cumbersome, and expensive to implement. Using one waveguide to carry both radio frequency (RF) signals in opposite directions has proved impractical because scattering by conventional RF couplers results in unacceptable interference between oppositely directed signals. Also, available RF waveguides do not have sufficient bandwidth for many applications, thus necessitating the use of separate waveguides for different frequency bands of interest. To maintain constant phase tracking between two different waveguides is a difficult engineering task. To maintain phase tracking among a plurality of waveguides is horrendously difficult.
In U.S. Pat. No. 3,953,727 (d'Auria et al.) a system for transmitting independent communication channels through a lightwave medium is disclosed. The system includes several laser diodes which supply infrared rays to quasi-transparent mirrors. These rays are focused into a fiber which conducts the rays to photodetectors. Communication can also occur in the reverse direction.
In U.S. Pat. No. 4,767,171 (Keil et al.) a transmission and reception module for a bi-directional communication network is provided. The network includes a housing containing a laser diode and a lens for focusing the output of the laser diode onto a first beam path. A second spherical lens is arranged on the first beam path for focusing the beam at a focal point at which an end of an optical fiber is positioned. A beam splitter which is at least partially transmissive to light of a first wavelength from the laser diode and partially reflective of light of a second wavelength being emitted from the fiber into a second beam path on which a detector is position is also provided. To improve the positioning, the first lens and the laser diode are mounted on a carrier member in a fixed relationship.
Also of general interest is U.S. Pat. No. 4,736,359 (Cohen et al.). This patent discloses a single fiber optical communication system which provides full-duplex, bi-directional transmission.